diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -1031,6 +1031,10 @@
   uint64_t Size = 0;
   uint32_t Align = 0;
 
+  const RecordDecl *D = RD->getDefinition();
+  if (D && D->isCompleteDefinition())
+    Size = CGM.getContext().getTypeSize(Ty);
+
   llvm::DINode::DIFlags Flags = llvm::DINode::FlagFwdDecl;
 
   // Add flag to nontrivial forward declarations. To be consistent with MSVC,
diff --git a/clang/test/CodeGenCXX/debug-info-class.cpp b/clang/test/CodeGenCXX/debug-info-class.cpp
--- a/clang/test/CodeGenCXX/debug-info-class.cpp
+++ b/clang/test/CodeGenCXX/debug-info-class.cpp
@@ -136,7 +136,7 @@
 // CHECK: [[C_DTOR]] = !DISubprogram(name: "~C"
 
 // CHECK: [[D:![0-9]+]] = !DICompositeType(tag: DW_TAG_structure_type, name: "D"
-// CHECK-NOT:              size:
+// CHECK-SAME:             size:
 // CHECK-SAME:             DIFlagFwdDecl
 // CHECK-NOT:              identifier:
 // CHECK-SAME:             ){{$}}
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -1013,8 +1013,10 @@
       Tag == dwarf::DW_TAG_class_type || Tag == dwarf::DW_TAG_structure_type ||
       Tag == dwarf::DW_TAG_union_type) {
     // Add size if non-zero (derived types might be zero-sized.)
+    // Ignore the size if it's a non-enum forward decl.
     // TODO: Do we care about size for enum forward declarations?
-    if (Size)
+    if (Size &&
+        (!CTy->isForwardDecl() || Tag == dwarf::DW_TAG_enumeration_type))
       addUInt(Buffer, dwarf::DW_AT_byte_size, None, Size);
     else if (!CTy->isForwardDecl())
       // Add zero size if it is not a forward declaration.
diff --git a/llvm/lib/Transforms/Scalar/SROA.cpp b/llvm/lib/Transforms/Scalar/SROA.cpp
--- a/llvm/lib/Transforms/Scalar/SROA.cpp
+++ b/llvm/lib/Transforms/Scalar/SROA.cpp
@@ -4534,15 +4534,15 @@
 
         // The alloca may be larger than the variable.
         auto VarSize = DbgDeclare->getVariable()->getSizeInBits();
-        if (VarSize) {
-          if (Size > *VarSize)
-            Size = *VarSize;
-          if (Size == 0 || Start + Size > *VarSize)
-            continue;
-        }
+        assert(VarSize &&
+               "Any variable with a location must have a type with a size");
+        if (Size > *VarSize)
+          Size = *VarSize;
+        if (Size == 0 || Start + Size > *VarSize)
+          continue;
 
         // Avoid creating a fragment expression that covers the entire variable.
-        if (!VarSize || *VarSize != Size) {
+        if (*VarSize != Size) {
           if (auto E =
                   DIExpression::createFragmentExpression(Expr, Start, Size))
             FragmentExpr = *E;
diff --git a/llvm/test/DebugInfo/X86/struct-fwd-decl.ll b/llvm/test/DebugInfo/X86/struct-fwd-decl.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/DebugInfo/X86/struct-fwd-decl.ll
@@ -0,0 +1,21 @@
+; RUN: llc -O0 -mtriple=x86_64-unknown-linux %s -o %t -filetype=obj
+; RUN: llvm-dwarfdump -debug-info %t | FileCheck %s
+; Test that size is not emitted for class declarations in DWARF, even if it exists.
+
+@s = global i16 0, align 2, !dbg !0
+
+!llvm.dbg.cu = !{!4}
+!llvm.module.flags = !{!7}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = !DIGlobalVariable(name: "s", scope: null, file: !2, line: 2, type: !3, isLocal: false, isDefinition: true)
+!2 = !DIFile(filename: "foo.cpp", directory: "/tmp")
+!3 = !DICompositeType(tag: DW_TAG_structure_type, name: "S", file: !2, line: 1, size: 16, align: 16, flags: DIFlagFwdDecl)
+!4 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !2, isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !5, retainedTypes: !5, globals: !6, imports: !5)
+!5 = !{}
+; CHECK: DW_TAG_structure_type
+; CHECK-NEXT: DW_AT_name
+; CHECK-NOT: DW_AT_byte_size
+; CHECK-NEXT: DW_AT_declaration
+!6 = !{!0}
+!7 = !{i32 1, !"Debug Info Version", i32 3}